The present invention concerns tandem master cylinders, in particular those destined to equip hydraulic brake circuits of motor vehicles, and it concerns more particularly a method of adjusting these to reduce their lost motion.
A conventional tandem master cylinder comprises a primary piston and a secondary piston sealingly sliding in a bore in order to define therein two pressure chambers each supplying a hydraulic circuit during the operation of the primary piston. A spring is placed in each pressure chamber.
Conventionally the preload of the primary spring is greater than that of the secondary spring and the length of the primary spring is limited in extension.
Each of the pressure chambers is supplied with low pressure fluid, on the one hand by a first orifice connecting a low pressure fluid reservoir to the corresponding chamber via a non-return type sealing ring, as well as by a second orifice called an expansion hole, of smaller size and positioned to be closed when the piston moves.
By way of convention, the elements closer to the brake pedal will be called primary and the elements further away will be called secondary.
During the operation of such a master cylinder, either by the push rod of a brake-booster, or directly by the brake pedal, initially the force exerted on the primary piston has the effect of equalizing the preload of the spring associated with the secondary piston and overcoming friction due to the sealing rings. In this first phase of operation of the master cylinder, the force exerted on the primary piston does not engender any hydraulic pressure in a brake circuit. Next, the force exerted on the primary piston continues to move forward the secondary piston whose movement permits closing the expansion hole. At that moment, the hydraulic pressure in the secondary circuit starts to rise and the corresponding piston therefore presents resistance to its advance.
This causes compression of the primary spring; the primary expansion hole having been closed at the beginning of the stroke by the primary piston, the hydraulic pressure increases in the corresponding chamber.
The lost motion that it is desired to adjust here is the distance that the secondary piston has to be moved to close the secondary expansion hole.
Depending on the manufacturing tolerances of the various elements comprising the master cylinder, it will be seen therefore that the lost motion can vary during a production run. In addition, in order to obtain braking action as rapidly as possible after beginning the operation of the master cylinder, it is desirable that its lost motion should be as little as possible.
A complicated method for adjustment of the lost motion of a master cylinder by inserting a spacer and adjusting the length of an adjusting screw is known from DE-A-38 19737. This method presents many disadvantages which make it impracticable, such as the need to adjust the lost motion of the secondary piston after having disassembled the master cylinder and before replacing the pistons and springs in the cylinder. This therefore results in an imprecise adjustment which is carried out in accordance with theoretical calculations and which cannot take into account the actual position of the secondary expansion hole and the various manufacturing tolerances. It also results in unreasonable manufacturing costs.
However, as described above, it is the lost motion of the secondary piston which it is necessary to minimize accurately because the stroke of the primary piston is clearly greater than that of the secondary piston, and because the pressure in the primary chamber cannot rise while the secondary expansion hole is open.